It is necessary to control the lighting system of a trailer in response to the lighting system of a towing vehicle. Devices for interfacing the lighting systems of towing vehicles and their respective trailers are well known. Most trailer lighting systems have combined the turn and brake signals in a single lamp with a single filament. Most modern day towing vehicles have separate turn signal and brake lamps. Thus, some kind of adapter circuitry is needed in order to interface the two systems, which would otherwise be incompatible.
Many devices known in the art accomplish this interface by using some kind of logic and switching circuitry. In essence the trailer lighting system is wired in parallel to the vehicle lighting system. Such a system uses the same power signal to illuminate both the trailer lamps and the towing vehicle lamps. To ensure that the signal, which reaches the trailer lamps, will be sufficient to adequately illuminate them, the original power signal through the vehicle lamps must have an increased amount of current. There are many disadvantages to such a system. For example, the flashers on the towing vehicle must be replaced with heavy-duty flashers to endure the increased current through them.
Devices have been developed that illuminate the vehicle and trailer lamps with separate power signals. This can be accomplished through the use of solid state switching devices, such as transistors, relay coils, or other types of signal detection devices. However, the known devices have many disadvantages. For example, they are susceptible to short circuit conditions, or are unable to disable a trailer lamp when both the brakes and a turn signal are engaged.